ASP.NET C sharp Select mit SUM von TextBox?

        protected void GridView1_SelectedIndexChanged1(object sender, GridViewSelectEventArgs e)
        {
            txt_ID.Text = GridView1.Rows[e.NewSelectedIndex].Cells[0].Text;
        }
        protected void Button2_Click(object sender, EventArgs e)
        {
            string ObjektID = txt_ID.Text;
            SqlConnection conn2 = new SqlConnection(ConfigurationManager.ConnectionStrings["SqlDataSource1"].ToString());  
            SqlCommand cmd2 = new SqlCommand();
            SqlCommand cmd3 = new SqlCommand("SELECT SUM(Summe) As SummeXY, ItemObjektID FROM Item WHERE ItemObjektID LIKE @ID GROUP BY ItemObjektID");  
            cmd3.Parameters.Add("@ID", SqlDbType.Int).Value = Convert.ToInt32(ObjektID);  

            cmd2.CommandText = "UPDATE Objekt SET Summe = @Sum WHERE ObjektID= @ID";  
            try
            {
                conn2.Open();
                cmd3.Connection = conn2;
                object result = cmd3.ExecuteScalar();
                cmd2.Connection = conn2;
                cmd2.Parameters.Add("@ID", SqlDbType.Int).Value = Convert.ToInt32(ObjektID);  
                cmd2.Parameters.Add("@Sum", SqlDbType.BigInt).Value = Convert.ToInt64(result);  
                cmd2.CommandType = CommandType.Text;
                cmd2.ExecuteNonQuery();
            }

            catch (System.Data.SqlClient.SqlException ex)
            {
                string msg = "Update Error:";  
                msg += ex.Message;
                throw new Exception(msg);
            }

            finally
            {
                conn2.Close();
            }